Active material for battery and battery having the same

ABSTRACT

An active material for a battery includes an electrochemically reversibly oxidizable and reducible base material selected from the group consisting of a metal, a lithium-containing alloy, a sulfur-based compound, and a compound that can reversibly form a lithium-containing compound by a reaction with lithium ions and a surface-treatment layer formed on the base material and comprising a compound of the formula MXO k , wherein M is at least one element selected from the group consisting of an alkali metal, an alkaline earth metal, a group 13 element, a group 14 element, a transition metal, and a rare-earth element, X is an element that is capable of forming a double bond with oxygen, k is a numerical value in the range of 2 to 4.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/507,547, filed Aug. 22, 2006, now U.S. Pat. No. 7,713,659 nowallowed, which is a continuation of U.S. application Ser. No. 09/995,868filed Nov. 29, 2001, issued U.S. Pat. No. 7,135,251, which claims thebenefit under 35 U.S.C. §119(e) of the U.S. Provisional Application Ser.No. 60/297,783, entitled “ACTIVE MATERIAL FOR BATTERY AND METHOD FORPREPARING SAME”, filed Jun. 14, 2001, and 60/304,793, of the same title,filed Jul. 13, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active material for a battery and amethod of preparing the same, and more specifically to an activematerial for a battery with excellent electrochemical characteristicsand thermal stability, and a method of preparing the same.

2. Description of the Related Art

Recently, in relation to trends toward more compact and lighter portableelectronic equipment, there has been a growing need to develop a highperformance and large capacity battery to be used for electric power forportable electronic equipment. Also, there has been extensive researchon batteries with good safety characteristics and low cost.

Generally, batteries are classified as primary batteries that can beused only once and secondary batteries that are rechargeable. Primarybatteries include manganese batteries, alkaline batteries, mercurybatteries, silver oxide batteries and so on, and secondary batteriesinclude lead-acid storage batteries, Ni-MH (nickel metal hydride)batteries, nickel-cadmium batteries, lithium metal batteries, lithiumion batteries, lithium polymer batteries and lithium-sulfur batteries.

These batteries generate electric power by using materials capable ofelectrochemical reactions at positive and negative electrodes. Factorsthat affect battery performance characteristics such as capacity, cyclelife, power capability, safety and reliability, include electrochemicalproperties and thermal stability of active materials that participate inelectrochemical reactions at the positive and negative electrodes.Therefore, research to improve the electrochemical properties andthermal stability of the active materials at the positive and negativeelectrodes continues.

Among the active materials currently being used for negative electrodesof batteries, lithium metal provides both high capacity because it has ahigh electric capacity per unit mass and high voltage due to arelatively high electronegativity. However, since it is difficult toassure the safety of a battery using lithium metal, other materials thatcan reversibly deintercalate and intercalate lithium ions are being usedextensively for the active material of the negative electrodes inlithium secondary batteries.

Lithium secondary batteries use materials that reversibly intercalate ordeintercalate lithium ions during charge and discharge reactions forboth positive and negative active materials, and contain organicelectrolyte or polymer electrolyte between the positive electrode andthe negative electrode. This battery generates electric energy fromchanges of chemical potential during the intercalation/deintercalationof lithium ions at the positive and negative electrodes.

Lithium metal compounds of a complex formula are used as the positiveactive material of the lithium secondary battery. Typical examplesinclude LiCoO₂, LiMn₂O₄, LiNiO₂, LiNi_(1-x)Co_(x)Co₂ (0<x<1), LiMnO₂ anda mixture of these compounds. Manganese-based positive active materialssuch as LiMn₂O₄ or LiMnO₂ are the easiest to synthesize, less costlythan the other materials, and environmentally friendly. However, thesemanganese-based materials have relatively low capacity. LiCoO₂ has goodelectric conductivity, high battery voltage and excellent electrodecharacteristics. This compound is presently the most popular materialfor positive electrodes of commercially available Li-ion batteries.However, it is relatively expensive and has low stability duringcharge-discharge at a high rate. LiNiO₂ is currently the least costly ofthe positive active materials mentioned above and has a high dischargecapacity, but it is difficult to synthesize and the least stable amongthe compounds mentioned above.

The above active materials are lithiated intercalation compounds inwhich stability and capacity of active material is determined by thenature of intercalation/deintercalation reactions of lithium ions. Asthe charging potential increases, the amount of Li deintercalationincreases, thus increasing the electrode capacity, but thermal stabilityof the electrode decreases steeply due to its structural instability.When the interior temperature of the battery increases in the fullycharged state, the bonding energy between the metal ions and the oxygenof the active material decreases, releasing oxygen when a temperatureabove a threshold value is reached. For example, LiCoO₂ active materialin a charged state has the formula Li_(1-x)CoO₂, where 0<x<1. Becausethe active material having the above structural formula is unstable,especially when x>0.5, if the interior temperature of the batteryincreases beyond the threshold value, oxygen gas (O₂) is released. Sincethe reaction of this oxygen with organic electrolyte in the battery ishighly exothermic, a thermal runaway situation may be created in thebattery, and this may cause an explosion in the battery. Therefore, itis desirable to control the threshold temperature and the amount ofexothermic heat evolved from the reaction in order to improve the safetyof the battery.

One way of controlling the threshold temperature and the amount ofexothermic heat is controlling the surface area of the active materialthrough particle size control, which is usually achieved by pulverizingand sieving the active material. The smaller the particle size, i.e. thelarger the surface area, the better the battery performance, inparticular the power capability, i.e. capacity values and dischargevoltages at low temperatures and at high rates. However, battery safety,cycle life and self-discharge become worse as the particle sizedecreases. Because of these conflicting factors, there is a practicallimitation in controlling the threshold temperature and heat evolutionrate through particle size alone.

In order to improve stability of active material itself duringcharge-discharge, it has been suggested to dope other elements into theNi-based or Co-based lithium oxide. For example, U.S. Pat. No. 5,292,601discloses Li_(x)MO₂ (where M is at least one element selected from Co,Ni and Mn; and x is 0.5 to 1) as an improved material over LiCoO₂.

Another attempt to improve stability includes modifying the surface ofthe active material. Japanese Patent Laid-Open No. Hei 9-55210 disclosesthat lithium nickel-based oxide is coated with alkoxide of Co, Al and Mnand is heat-treated to prepare a positive active material. JapanesePatent Laid-Open No. Hei 11-16566 discloses lithium-based oxide coatedwith a metal and/or an oxide thereof. The metal includes Ti, Sn, Bi, Cu,Si, Ga, W, Zr, B or Mo. Japanese Patent Laid-Open No. Hei 11-185758discloses coating a surface of lithium manganese oxide with a metaloxide by using a co-precipitation process and heat-treating the same toprepare a positive active material.

However, the above methods did not solve the fundamental problemsassociated with the safety of the battery: The threshold temperaturewherein the active material prepared according to the above methodsbegins to react with an electrolyte, that is, the decompositiontemperature, at which oxygen bound to metal of the active materialbegins to be released (exothermic starting temperature, T_(s)) does notshift sufficiently to a higher temperature and the amount of releasedoxygen (the value related to the exothermic heat) does not decreasesufficiently by the methods described above.

The structural stability of positive active material having thecomposition of Li_(1-x)MO₂ (M═Ni or Co) during charging is stronglyinfluenced by the value of x. That is, when 0<x<0.5, cyclic stability issteadily and stably maintained, but when x is greater than or equal to0.5, phase transition occurs from a hexagonal phase to a monoclinicphase. This phase transition causes an anisotropic volume change, whichinduces development of micro-cracks in the positive active material.These micro-cracks damage the structure of the active material, and thusthe battery capacity decreases dramatically and the cycle life isreduced. Therefore, when anisotropic volume change is minimized, thecapacity and the cycle life of the battery are improved.

In order to increase structural stability of positive active material,U.S. Pat. No. 5,705,291 discloses a method in which a compositioncomprising borate, aluminate, silicate or mixtures thereof was coatedonto the surface of a lithiated intercalation compound, but it still hasa problem with structural stability.

In the above description, positive active materials of lithium secondarybatteries and related examples of developments were explained. Recently,in relation to the tendency to develop portable electronic equipmentthat is more compact and lightweight, other types of batteries have thesame demands for an active material that guarantees battery performance,safety and reliability. Research and development is thereforeaccelerated on electrochemical properties and thermal stability ofpositive active materials to ensure improved performance, safety andreliability of batteries.

SUMMARY OF THE INVENTION

In order to solve the problems stated above, it is an object of thepresent invention to provide an active material for a battery with goodelectrochemical characteristics, such as capacity and cycle life.

It is another object to provide an active material for a battery withgood thermal stability.

It is still another object to provide a method of preparing an activematerial with good manufacturing productivity and an economicalpreparation process.

In order to accomplish these and other objects, the present inventionprovides an active material for a battery having a surface treatmentlayer comprising the compound having the formula (I):MXO_(k)  (1)

wherein M is at least one selected from the group consisting of analkali metal, an alkaline earth metal, a group 13 element, a group 14element, a transition metal and a rare-earth element; X is an elementthat can form a double bond with oxygen; and k is a numerical value inthe range of 2 to 4.

The present invention also provides a process for preparing an activematerial for a battery comprising: preparing a coating liquid by addinga compound comprising an element X that is capable of forming a doublebond with oxygen, and a compound comprising at least one from the groupconsisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement, to water; adding active material to the coating liquid to coatthe material with the suspension; and heat-treating the coated activematerial to form a surface-treatment layer comprising the compound ofthe formula (1).

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 shows voltage and capacity characteristics at various C-ratedischarges of half-cells of Example 1 of the present invention andComparative Example 1.

FIG. 2 shows voltage and capacity characteristics of half-cellsaccording to Example 1 of the present invention and Comparative Example1 at various cycle numbers at a 1C rate of discharge.

FIG. 3A shows charge and discharge curves of a half-cell according toExample 5 of the present invention at rates of 0.2C, 0.5C, and 1C in thevoltage range of 4.3V to 2.75V.

FIG. 3B is an expanded view of a portion of FIG. 3A.

FIG. 4 shows charge and discharge curves of a half-cell according toExample 2 of the present invention at rates of 0.2C, 0.5C and 1C in thevoltage range of 4.3V to 2.75V.

FIG. 5 shows charge and discharge curves of a half-cell according toExample 12 of the present invention at rates of 0.2C, 0.5C and 1C in thevoltage range of 4.3V to 2.75V.

FIG. 6 shows voltage and discharge capacities of half-cells according toExamples 5 to 7 of the present invention and Comparative Example 1 at 1Crate of discharge.

FIG. 7 shows charge and discharge curves of half-cells according toExample 1 of the present invention and Comparative Example 1 at rates of0.2C, 0.5C and 1C in the voltage range of 4.6V to 2.75V.

FIG. 8 shows discharge curves of a half-cell according to Example 1 ofthe present invention at rates of 0.2C, 0.5C and 1C in the voltage rangeof 4.95V to 2.75V.

FIG. 9 shows cycle life characteristics at 1C rate of half-cells ofExample 1 of the present invention and Comparative Example 3.

FIG. 10 shows discharge curves of a Li-ion cell comprising activematerial of Example 15 of the present invention at rates of 0.5C, 1C and2C in the voltage range of 4.2V to 2.75V.

FIGS. 11A to 11E show the results of elemental analyses of activematerials prepared according to Example 1 of the present invention usingScanning Transmission Electron Microscopy (STEM).

FIG. 12 shows the results of elemental analyses for components of asurface-treatment layer of active material prepared according to Example1 of the present invention using Auger Spectroscopy.

FIGS. 13A and 13B show the results of elemental distribution analysesfor a cross-sectional view of active material prepared according toExample 1 of the present invention, by line scanning with Electron ProbeMicro Analysis (EPMA).

FIG. 14 shows the results of an analysis of a surface-treatment layer ofactive material of Example 1 of the present invention, by X-rayphotoelectron spectroscopy (XPS).

FIGS. 15A and 15B show cyclic voltammograms of active materialsaccording to Example 5 of the present invention and Comparative Example1.

FIG. 16 shows diffusion coefficients (D_(Li+)) of lithium ions of activematerials of Example 5 according to the present invention andComparative Example 1.

FIGS. 17A and 17B show changes of lattice constants of the activematerial in various states of charge of half-cells prepared according toExample 5 of the present invention and Comparative Example 1, in thevoltage range of 4.6V to 4.25V.

FIG. 18 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained after charging half-cells prepared accordingto Example 5 of the present invention and Comparative Example 1, at4.3V.

FIG. 19 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained after charging half-cells prepared accordingto Examples 5 to 7 of the present invention and Comparative Example 1,at 4.3V.

FIG. 20 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained after overcharging half-cells preparedaccording to Example 1 of the present invention and Comparative Example1, at 4.6V.

FIG. 21 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained after overcharging half-cells preparedaccording to Example 8 of the present invention and Comparative Example2, at 4.6V.

FIG. 22 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained after overcharging half-cells preparedaccording to Example 8 of the present invention and Comparative Examples2 and 4, at 5V.

FIG. 23 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained after overcharging half-cells preparedaccording to Examples 1 and 9 of the present invention and ComparativeExamples 1 and 4, at 5V.

FIG. 24 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained after overcharging half-cells preparedaccording to Example 10 of the present invention and Comparative Example4, at 5V.

FIG. 25 shows the results of Differential Scanning Calorimetry (DSC) ofactive materials obtained after overcharging half-cells preparedaccording to Examples 12 and 13 of the present invention and ComparativeExample 5, at 4.6V.

FIG. 26 shows charge voltage and cell temperature of a Li-ion cellcomprising active material prepared in Example 1 when the cell isovercharged at 1C rate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The active material for a battery of the present invention comprises asurface-treatment layer comprising a compound with the formula (I) onthe surface thereof:MXO_(k)  (1)wherein M is at least one element selected from the group consisting ofan alkali metal, an alkaline earth metal, a group 13 element, a group 14element, a transition metal, and a rare-earth element; X is an elementthat is capable of forming a double bond with oxygen; and k is anumerical value in the range of 2 to 4.

The group 13 element (according to the new IUPAC agreement) refers tothe element group including Al of the Periodic Table. The group 14element (according to the new IUPAC agreement) refers to the elementgroup including Si of the Periodic Table. In the preferred examples ofthe present invention, M includes Na, K, Mg, Ca, Sr, Ni, Co, Si, Ti, B,Al, Sn, Mn, Cr, Fe, V, Zr, or a combination thereof, and X is P, S, W ora combination thereof. The element X forms a double bond with oxygen,which means a classical chemical bonding. For example, in classicalChemistry, when X bonds with four oxygen elements, it means one doublebond and three single bonds. However, in modern Chemistry, it means thatX bonds with 1.25 oxygens because of delocalization of electrons.

The amount of element M of the compound with the formula (I) of thepresent invention is 0.1 to 15% by weight, preferably 0.1 to 6% byweight of the active material. Also, the amount of element X that iscapable of forming a double bond with oxygen of the compound having theformula (I) is 0.1 to 15% by weight, preferably 0.1 to 6% by weight ofthe active material. When the amount of M or X present in the surface ofthe active material is not in the above range, electrochemicalcharacteristics at a high rate are not improved and the thermalstability is not improved by the coating.

The thickness of the surface-treatment layer of the present invention ispreferably 0.01 to 2 μm, and more preferably 0.01 to 1 μm. While otherthicknesses are possible, if the thickness of the surface-treatmentlayer is less than 0.01 μm, the effect obtained from thesurface-treatment layer may not be realized. In contrast, if thethickness is more than 2 μm, the capacity of the battery isdeteriorated.

In the case that the surface-treated active material is a lithiatedintercalation compound, a solid-solution compound between the lithiatedintercalation compound and the MXO_(k) compound with the formula (I) isformed on the surface of the active material in addition to the MXO_(k)compound. In this case, a surface-treatment layer of the active materialcomprises both the solid-solution compound and the MXO_(k) compound. Thesolid-solution compound comprises Li, M′ (M′ is at least one selectedfrom the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and arare-earth element, that originate from the lithiated intercalationcompound), M (M is at least one selected from the group consisting of analkali metal, an alkaline earth metal, a group element, a group 14element, a transition metal, and a rare-earth element), X (an elementcapable of forming a double bond with oxygen), and O (oxygen).

When the surface-treatment layer comprising the solid-solution compoundand the MXO_(k) compound on the surface of these intercalation compoundsis formed, the elements M and X have a concentration gradient from thesurface of the active material toward the center of the active materialparticle grain. That is, M and X have a high concentration at thesurface of the active material and the concentration gradually decreasestoward the inside of the particle.

According to the preferable examples of the present invention, theactive material for a battery comprising a lithiated intercalationcompound and a surface-treatment layer comprising the solid-solutioncompound with Al and P, and AlPO_(k) (k is 2 to 4) is provided.

The surface treatment technique of the active material with the MXO_(k)compound of the present invention may be used for all batteries, and iseffective in improving the performance characteristics of both activematerials for the positive electrodes as well as the negativeelectrodes. The surface-treated active material includes materials thatcan undergo reversible electrochemical oxidation-reduction reactions.The electrochemically oxidizable and reducible material includes ametal, a lithium-containing alloy, sulfur-based compounds, compoundsthat can reversibly form lithium-containing compounds by a reaction withlithium ions, all materials that can reversiblyintercalate/deintercalate lithium ions (lithiated intercalationcompounds), although the present invention is not limited thereto.

The metal includes lithium, tin or titanium. The lithium-containingalloy includes a lithium/aluminum alloy, a lithium/tin alloy, or alithium/magnesium alloy. The sulfur-based compound which is the positiveactive material of the lithium-sulfur battery includes a sulfur element,Li₂S_(n) (n≧1), an organic sulfur compound and a carbon-sulfur polymer((C₂S_(x))_(n) where x=2.5 to 50 and n≧2). The compound that canreversibly form a lithium-containing compound by a reaction with lithiumions includes silicon, titanium nitrate or tin oxide (SnO₂).

The active material that can reversibly intercalate/deintercalatelithium ion (lithiated intercalation compounds) includes carbon-basedmaterial, lithium-containing metal oxides, and lithium-containingchalcogenide compounds. The carbon-based material can be non-crystallinecarbon, crystalline carbon, or a mixture thereof. Examples of thenon-crystalline carbon includes soft carbon (low temperature calcinatedcarbon), and hard carbon (high temperature calcinated carbon). Examplesof crystalline carbon include natural graphite or artificial graphitewhich are plate, sphere or fiber shape.

The lithium-containing metal oxide and lithium-containing chalcogenidecompound has a monoclinic, hexagonal or cubic structure as a basicstructure.

A conventional lithium-containing compound (lithium-containing metaloxide and lithium-containing chalcogenide compound) can be used as thelithiated intercalation compound of the present invention, andpreferable examples are as follows:Li_(x)Mn_(1-y)M′_(y)A₂  (2)Li_(x)Mn_(1-y)M′_(y)O_(2-z)B_(z)  (3)Li_(x)Mn₂O_(4-z)B_(z)  (4)Li_(x)Mn_(2-y)M′_(y)A₄  (5)Li_(x)Co_(1-y)M′_(y)A₂  (6)Li_(x)CO_(1-y)M′_(y)O_(2-z)B_(z)  (7)Li_(x)Ni_(1-y)M′_(y)A₂  (8)Li_(x)Ni_(1-y)M′_(y)O_(2-z)B_(z)  (9)Li_(x)Ni_(1-y)Co_(y)O_(2-z)B_(z)  (10)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)A_(α)  (11)Li_(x)Ni_(1-y-z)CO_(y)M′_(z)O_(2-α)B_(α)  (12)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)A_(α)  (13)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)O_(2-α)B_(α)  (14)

wherein 0.95≦x≦1.1, 0≦y≦0.5, 0≦z≦0.5, and 0<α≦2;

M′ is at least one element selected from the group consisting of Al, Ni,Co, Mn, Cr, Fe, Mg, Sr, V, and a rare-earth element;

A is at least one element selected from the group consisting of O, F, Sand P; and

B is at least one element selected from the group consisting of F, S andP.

The average particle size of these lithiated intercalation compounds ispreferably 1 to 20 μm, more preferably 3 to 15 μm.

In the present invention, a surface-treatment layer comprising theMXO_(k) compound is formed on the surface of the active material. Whenthe active material is a lithiated intercalation compound, asurface-treatment layer comprising a solid-solution compound includingLi, M′ (M′ is at least one selected from the group consisting of Al, Ni,Co, Mn, Cr, Fe, Mg, Sr, V, and a rare-earth element that originate fromlithiated intercalation compounds), M (M is at least one selected fromthe group consisting of an alkali metal, an alkaline earth metal, agroup 13 element, a group 14 element, a transition metal, and arare-earth element), X (an element capable of forming a double bond withoxygen) and O; as well as the MXO_(k) compound, is formed.

As a general rule, the capacity of a battery cell using the activematerial with a high tap density is greater than one using a similaractive material having a low tap density. Therefore, an improved tapdensity of the active material is generally desired for a battery cell.The active materials surface-treated according to the present inventionshow a much higher tap density than the corresponding equivalent activematerial without surface-treatment, indicating that thesurface-treatment facilitates compaction of the active material powder.The tap density of the active material of the present invention ismaintained at 1 to 3 g/cc, thus it increases the capacity of the batterycell. According to the preferable example of the present invention, thetap density of the active material is more than about 2 g/cc.

The active materials, with the surface-treated according to the presentinvention, also produces electrodes having a much higher electrodedensity, meaning more active material per unit volume, than thecorresponding active material without surface-treatment, when theelectrodes are fabricated by a conventional electrode fabricationprocess used by the Li-ion battery industries which involves a coatingonto a current collector of an active material slurry comprising aconductive agent, a binder and a solvent in addition to the activematerial. When the electrodes are compacted by compression, theelectrodes containing the surface-treated active material compact wellwithout micro-cracks in the active material powder, while the electrodecontaining bare active material shows micro-cracks in the activematerial powder. The surface-treatment of the active material of thepresent invention might possibly provide a lubricating effect on thesurface of the powder for improved compaction of the active material.

The most important factor affecting safety of a battery is thereactivity of the active material in a charged state at its surfacetoward the electrolyte. For example, one of lithiated intercalationcompounds, LiCoO₂, has a structure of α-NaFeO₂, while it has a structureof Li_(1-x)CoO₂ during charge and discharge cycles. When it is chargedat a voltage over 4.93V, Li is completely removed, and it has astructure of a hexagonal type of CdI₂. In such a lithium metal oxide, asthe amount of lithium decreases, thermal stability decreases and itbecomes a stronger oxidant. When a battery containing LiCoO₂ activematerial is fully charged, the active material composition becomesLi_(1-x)CoO₂ where x is greater than or equal to 0.5. Such a compositionbecomes unstable as the battery temperature rises, i.e., the oxygenbound with metal, that is cobalt, is released to form gaseous O₂. Thereleased oxygen might react with electrolyte inside the battery,possibly leading to an explosion. Therefore, the oxygen-releasingtemperature (exothermic reaction starting temperature) and the amount ofexothermic heat released by the reaction are important factors todetermine the safety of the battery. Such thermal stability can beevaluated from DSC (Differential Scanning Calorimetry) curves bydetermining the starting temperature of the exothermic reaction and theheat of reaction.

Unlike conventional active material, for the active material that issurface-treated with the MXO_(k) compound of the present invention, theDSC exothermic peak is almost negligible in size since the MXO_(k)compound inhibits reaction with electrolyte. Therefore, the activematerial of the present invention is substantially improved in thermalstability over the conventional surface-untreated active material.

The surface-treatment layer comprising the MXO_(k) compound of thepresent invention may be applied to the active material for a primarybattery such as a manganese battery, an alkaline battery, a mercurybattery, a silver oxide battery, as well as to the active material for asecondary battery such as a lead-acid storage battery, a Ni-MH (nickelmetal hydride) battery, a nickel-cadmium battery, a lithium metalbattery, a lithium ion battery, a lithium polymer battery and alithium-sulfur battery. The structures of such batteries, including alithium secondary battery shell, are well known, as indicated, e.g., byU.S. Pat. No. 5,753,387, the disclosure of which is incorporated byreference herein. The active material having the surface-treatment layeris used in at least one of a positive electrode and a negative electrodeof the above batteries.

The process for preparing active material having the surface-treatmentlayer of MXO_(k) compound is as follows.

First, the coating liquid is prepared by reacting a compound comprisingX (an element that is capable of forming a double bond with oxygen) witha compound comprising M (an alkali metal, an alkaline earth metal, agroup 13 element, a group 14 element, a transition metal, a rare-earthelement or a combination thereof) in water. In this invention, “coatingliquid” refers to a homogeneous suspension or a solution.

Since water is used as a solvent in the coating liquid, the presentprocess is advantageous over the process using an organic solvent forthe process cost-reduction.

The choice of the compound type comprising element (X) has no particularlimitation as long as the compound is soluble in water. For example,when X is P, it can be diammonium hydrogen phosphate ((NH₄)₂HPO₄), P₂O₅,H₃PO₄, or Li₃PO₄. The content of the compound comprising X is preferably0.01 to 30% by weight, more preferably 0.1 to 20% by weight of the totalweight of the coating liquid.

The element (M) used for the coating liquid is an alkali metal, analkaline earth metal, a group 13 element, a group 14 element, atransition metal, a rare-earth element or a combination thereof. Thepreferable examples of these elements are Al, Ni, Co, Zr, Mn, Cr, Fe,Mg, Sr, V, or a combination thereof. The choice of the compound typecomprising these elements has no particular limitation as long as thecompound is soluble in water. The preferred examples are a nitrate andan acetate. The amount of the compound comprising an alkali metal, analkaline earth metal, a group 13 element, a group 14 element, atransition metal, a rare-earth element or a combination thereof ispreferably 0.01 to 30% by weight, more preferably 0.1 to 20% by weightof the weight of the coating liquid.

The coating liquid as prepared above is used to coat the activematerial. The coating may be achieved by simply adding a predeterminedamount of the coating liquid to a given amount of the active materialpowder followed by a through mixing and optionally drying, although thepresent invention is not limited to this method.

Then, the coated active material is heat-treated at 100 to 700° C.,preferably at 100 to 500° C. for 1 to 20 hours. If the heat-treatmentprocess is over-done, an AlPO_(k) (k is 2 to 4) compound diffuses intothe inside of the active material resulting in a battery capacitydecrease. Before the heat-treatment process, a separate drying processthat dries the coated liquid may be used. In the present invention,since the heat-treatment process is made at a lower temperature and fora shorter time than a prior-art process using organic solvent, whichrequires a higher calcination temperature and a longer calcination time,it reduces cost during large-scale production.

In the prior-art process, a sieving process step is required sinceparticle agglomerations occur usually due to a high calcinationtemperature. However, in the process of the present invention, such asieving process is not required since the calcination temperature issignificantly reduced resulting in negligible particle agglomerations.

The desired surface-treatment layer comprising the MXO_(k) compound isformed on the surface of the active material after the heat-treatmentprocess. When the active material being coated is a lithiatedintercalation compound, a solid-solution compound which is formed bycombination of the lithiated intercalation compound and the MXO_(k)compound may be formed between the layer of MXO_(k) compound and theactive material.

In forming the battery, the method includes preparing a coating liquidby adding a compound including an element that is capable of forming adouble bond with oxygen of lithium metal oxide, and a metal compoundcomprising at least one element from the group consisting of an alkalimetal, an alkaline earth metal, a group 13 element, a group 14 element,a transition metal, and a rare-earth element, to water. An activematerial is added to the coating liquid to coat the active material. Thecoated active material is heat treated to prepare an active materialhaving a surface-treatment layer comprising a compound having theformula (I):MXO_(k)  (1)

wherein M is at least one element selected from the group consisting ofan alkali metal, an alkaline earth metal, a group 13 element, a group 14element, a transition metal, and a rare-earth element, X is an elementthat is capable of forming a double bond with oxygen, and k is anumerical value in the range of 2 to 4. Next, a slurry comprising theactive material with the surface treatment layer is coated onto acurrent collector to prepare at least one of a positive and negativeelectrode which is used in the fabrication of a battery.

The present invention is further explained in more detail with referenceto the following examples. These examples, however, should not in anysense be interpreted as limiting the scope of the present invention.

Example 1

A coating liquid was prepared by adding 1 g of (NH₄)₂HPO₄ and 1.5 g ofaluminum nitrate (Al(NO₃)₃.9H₂O) to 100 ml of water. The resultingliquid was a homogeneous colloidal suspension of amorphous AlPO_(k)phase. After adding a 10 ml portion of the coating liquid to 20 g ofLiCoO₂ powder having an average particle diameter of 10 μm, it wasthoroughly mixed before drying at 130° C. for 30 minutes. The positiveactive material with the coating layer comprising a solid-solutioncompound including Al and P and the AlPO_(k) compound on the surface wasfurther heat-treated at 400° C. for 5 hours to obtain the desiredcoating. The total amount of Al and P was 1% by weight of the totalactive material weight.

The slurry for the positive electrode containing the positive activematerial as described, super P (conductive agent), and polyvinylidenefluoride (binder) in the weight ratio of 94/3/3 was prepared by mixingthem thoroughly in an N-methylpyrrolidone (NMP) solvent. The slurrycomposition comprising the positive active material was coated on an Alfoil at a thickness of about 300 μm, dried for 20 minutes at 130° C.,and pressed under a 1 ton pressure to make a positive electrode for acoin cell. A coin-typed half-cell was prepared by using this positiveelectrode and lithium metal as a counter electrode. For the electrolyte,1M LiPF₆ solution of mixed solvent of ethylene carbonate (EC) anddimethyl carbonate (DMC) in the volume ratio of 1:1 was used.

Example 2

A coin-typed half-cell was prepared by the same method as in Example 1,except that a ml portion of the coating liquid of Example 1 was added to20 g of LiCoO₂ with an average particle diameter of 10 μm. The totalamount of Al and P was 1.5% by weight of the total active materialweight.

Example 3

A coin-typed half-cell was prepared by the same method as in Example 1,except that a ml portion of the coating liquid of Example 1 was added to20 g of LiCoO₂ with an average particle diameter of 10 μm. The totalamount of Al and P was 2% by weight of the total active material weight.

Example 4

A coin-typed half-cell was prepared by the same method as in Example 1,except that a 10 ml portion of the coating liquid of Example 1 was addedto 20 g of LiCoO₂ with an average particle diameter of 5 μm. The totalamount of Al and P was 1% by weight of the total active material weight.

Example 5

A coin-typed half-cell was prepared by the same method as in Example 1,except that the heat-treatment time was 10 hours.

Example 6

A coin-typed half-cell was prepared by the same method as in Example 1,except that the heat-treatment step was made at 500° C. for 5 hours.

Example 7

A coin-typed half-cell was prepared by the same method as in Example 1,except that the heat-treatment step was made at 500° C. for 10 hours.

Example 8

A coin-typed half-cell was prepared by the same method as in Example 1,except that a 20 ml portion of the coating liquid of Example 1 was addedto 20 g of LiCoO₂ with an average particle diameter of 5 μm, and theheat-treatment step was made at 400° C. for 10 hours. The total amountof Al and P was 2% by weight of the total active material weight.

Example 9

A coin-typed half-cell was prepared by the same method as in Example 1,except that a 15 ml portion of the coating liquid of Example 1 was addedto 20 g of LiCoO₂ with an average particle diameter of 10 μm, and theheat-treatment step was made at 400° C. for 10 hours. The total amountof Al and P was 1.5% by weight of the total active material weight.

Example 10

A coin-typed half-cell was prepared by the same method as in Example 1,except that LiMn₂O₄ with an average particle diameter of 13 μm was usedinstead of LiCoO₂.

Example 11

A coin-typed half-cell was prepared by the same method as in Example 1,except that LiNi_(0.9)CO_(0.1)Sr_(0.002)O₂ with an average particlediameter of 13 μm was used instead of LiCoO₂.

Example 12

A coin-typed half-cell was prepared by the same method as in Example 1,except that LiNi_(0.8)Mn_(0.2)O₂ with an average particle diameter of 10μm was used instead of LiCoO₂ and the heat-treatment step was made at400° C. for 10 hours.

Example 13

A coin-typed half-cell was prepared by the same method as in Example 1,except that 20 g of LiNi_(0.8)Mn_(0.2)O₂ with an average diameter of 10μm was used instead of LiCoO₂, it was coated with 20 ml of coatingliquid prepared in Example 1 and the heat-treatment step was made at400° C. for 10 hours.

Example 14

A coin-typed half-cell was prepared by the same method as in Example 1,except that Li_(1.03)Ni_(0.69) Mn_(0.19)CO_(0.1)Al_(0.17)Mg_(0.07)O₂with an average diameter of 13 μm was used instead of LiCoO₂.

Example 15

A slurry containing positive active material was prepared by mixing thepositive active material of Example 1, super P (conductive agent), andpolyvinylidene fluoride (binder) in the weight ratio of 96/2/2 in amixing solvent of N-methylpyrrolidone (NMP). The positive electrode wasprepared using this slurry by the same method as in Example 1. A slurrycontaining negative active material was prepared by mixing artificialgraphite as a negative active material and polyvinylidene fluoride as abinder in the weight ratio of 90/10 in a mixing solvent of NMP. Thenegative electrode was prepared by casting the slurry containing thenegative active material on a Cu foil. A 930 mAh prismatic Li-ion cellwas fabricated using the positive and the negative electrodes. For theelectrolyte, 1M LiPF₆ solution of a mixed solvent of ethylenecarbonate/dimethyl carbonate in the volume ratio of 1/1 was used.

Example 16

A coin-typed half-cell was prepared by the same method as in Example 1,except that a 20 ml portion of the coating liquid of Example 1 was addedto 20 g of natural graphite. The total amount of Al and P was 2% byweight of the total weight of the total active material weight.

Example 17

A coin-typed half-cell was prepared by the same method as in Example 1,except that a 20 ml portion of the coating liquid of Example 1 was addedto 20 g of SnO₂. The total amount of Al and P was 2% of the total activematerial weight.

Example 18

A coin-typed half-cell was prepared by the same method as in Example 1,except that a 20 ml portion of the coating liquid of Example 1 was addedto 20 g of silicon (Si) active material. The total amount of Al and Pwas 2% by weight of the total active material weight.

Comparative Example 1

A coin-typed half-cell was prepared by the same method as in Example 1,except that LiCoO₂ with an average particle diameter of 10 μm was usedas the positive active material.

Comparative Example 2

A coin-typed half-cell was prepared by the same method as in Example 1,except that LiCoO₂ with an average particle diameter of 5 μm was used asthe positive active material.

Comparative Example 3

A coin-typed half-cell was prepared by the same method as in Example 1,except that aluminum nitrate (Al(NO₃)₃.9H₂O) was not added to thecoating liquid, and thus the active material coated with P₂O₅ on thesurface was prepared.

Comparative Example 4

A coin-typed half-cell was prepared by the same method as in Example 1,except that LiMn₂O₄ with an average particle diameter of 13 μm was usedas the positive active material.

Comparative Example 5

A coin-typed half-cell was prepared by the same method as in Example 1,except that LiNi_(0.8)Mn_(0.2)O₂ with an average particle diameter of 10μm was used as the positive active material.

Comparative Example 6

A coin-typed half-cell was prepared by the same method as in Example 1,except that LiNi_(0.9)CO_(0.1)Sr_(0.002)O₂ with an average particlediameter of 13 μm was used as the positive active material.

Comparative Example 7

A coin-typed half-cell was prepared by the same method as in Example 1,except that Li_(1.03)Ni_(0.69)Mn_(0.19)Co_(0.1)Mg_(0.07)O₂ with anaverage particle diameter of 13 μm was used as the positive activematerial.

Evaluation of Electrochemical Characteristics

Charge-discharge characteristics of the coin-typed half-cell of Example1 at 0.2C, 0.5C and 1C rates in the voltage range of 4.3V to 2.75V areshown in FIG. 1. For comparison, the characteristics of the cell ofComparative Example 1 are also shown. As seen in FIG. 1, the initialcapacity of the cell of Comparative Example 1 is much smaller at a highrate (1C) than those at low rates (0.2C and 0.5C). However, the initialcapacity of the cell of Example 1 is very high (152 mAh/g) even at thehigh rate. This value is close to those at low rates.

FIG. 2 shows capacity characteristics for cycling at 1C rate of thehalf-cells according to Example 1 and Comparative Example 1 in thevoltage range of 4.3V to 2.75V. In the Example 1, more than 99% ofinitial capacity was maintained after 30 charge-discharge cycles. Incontrast, the capacity of the cell of Comparative Example 1 decreasedsharply after 30 charge-discharge cycles. In addition, the averagedischarge voltage of the cell of Comparative Example 1 decreasedsignificantly with repeated cycling while the average value for Example1 showed a negligible change.

FIG. 3A shows the charge-discharge characteristics of a half-cellaccording to Example 5 at 0.2C, 0.5C and 1C rates in the voltage rangeof 4.3V to 2.75V. In order to confirm reproducibility ofcharge-discharge characteristics, powder of the positive active materialof Example 5 was synthesized in a large scale (1.5 kg batch size) andabout thirty cells were made to evaluate the charge-dischargecharacteristics. FIG. 3A represents the average values. FIG. 3B is anexpanded view of a portion of FIG. 3A. FIG. 3A shows that the initialcapacity at 1C rate was 150-152 mAh/g and the average voltage was about3.91V, which is similar to those at a low rate (0.2C). One noticeableobservation was that the discharge curve at 1C rate gradually approachesthat at 0.2C rate as the charge-discharge cycling at 1C rate proceeds.This means that there is a slight improvement rather than deteriorationof discharge capacity due to a decrease in internal resistance withcycling.

FIG. 4 shows the results of charge-discharge cycling of the cell ofExample 2 at 0.2C, 0.5C and 1C rates in the voltage range of 4.3V to2.75V. On repeated charge-discharge cycling, the average voltages at a1C rate approach those at 0.2C rate. This observation is also similar tothose of Example 12 as shown in FIG. 5. This observation indicates thatthe coating technique of the present invention is effective in improvingthe cell performance for Ni-based compounds as well as for Co-basedcompounds.

Discharge capacities of FIG. 1, FIG. 3A, FIG. 4 and FIG. 5 aresummarized as shown in Table 1.

TABLE 1 discharge capacities at various C-rate rates (unit: mAh/g)C-rate Com. Ex. 1 Ex. 1 Ex. 2 Ex. 5 Ex. 12 0.2C 154 159 159 161 170 0.5C151 156 156 159 161 1C 143 152 152 152 146

In order to evaluate the effects of heat-treatment temperature and timeon the electrochemical characteristics of cells, FIG. 6 shows theresults of charge-discharge cycling of the cells of Examples 5 to 7 at1C rate in the voltage range of 4.3V to 2.75V. The cells containingheat-treated positive active material powder at various heat-treatmenttemperatures and times showed improved high-rate characteristics overthose of the cell containing the conventional uncoated LiCoO₂(Comparative Example 1).

FIG. 7 shows the results of charge-discharge cycling of cells of Example1 and Comparative Example 1 at 0.2C, 0.5C and 1C rates in the voltagerange of 4.6V to 2.75V instead of 4.3V to 2.75V. Comparative Example 1shows that as the discharge rate increases, the discharge curve abruptlydeteriorates and high cell polarization is indicated. The capacity at 1Crate after 30 cycles was less than 70% of the initial capacity. On thecontrary, the cell of Example 1 showed remarkably improved 1C-ratecharacteristics; i.e., high discharge voltage and improved cycle lifeeven after charging at 4.6V.

FIG. 8 shows the results of charge-discharge cycling of cells of Example1 and Comparative Example 1 at 0.2C, 0.5C and 1C rates in the voltagerange of 4.95V to 2.75V. The cell of Comparative Example 1 did not showany measurable discharge capacity when it was charged at 4.95V, whilethe cell of Example 1 showed excellent discharge characteristics aftercharging at 4.95V, similar to the case of 4.6V charging.

As explained above, electrochemical characteristics of the cellcontaining positive active material of the present invention areexcellent because the solid-solution compound including Al and P, andthe AlPO_(k) (k is 2 to 4) compound which is formed on the surface ofthe lithiated intercalation compound probably improve conductivity oflithium ions and reduce surface polarization at high rates.

In order to verify the effect of the surface-treatment layer on theperformance of the cell, cycle life characteristics at 1C rate of thecells of Example 1 containing the active material that has thesurface-treatment layer comprising the solid-solution compound includingAl and P on the surface as well as the AlPO_(k) (k is 2 to 4) compoundand the cell of Comparative Example 3 containing the active materialconsisting of a P-containing layer derived from P₂O₅, are shown in FIG.9. Cycle life characteristics were measured by varying charge-dischargerate from 0.2C to 0.5C and 1C. As shown in FIG. 9, the cell capacity ofComparative Example 3 decreases rapidly with cycling, while the cellcapacity of Example 1 is maintained constant with cycling at the initialvalue.

FIG. 10 shows discharge voltage curves of a 930 mAh prismatic Li-ioncell prepared in Example 15 at discharge rates of 0.5C, 1C and 2C in thevoltage range of 4.2V to 2.75V. As shown in FIG. 10, the dischargecapacity of the cell at 2C rate was more than 95% of that at 0.5C rate.Therefore, Li-ion cells comprising the active material preparedaccording to the present invention have excellent cell performance,similar to those of the coin-type half-cells.

In order to evaluate the relationship between tap density and capacityof the active material of the present invention, electrodes prepared inExample 1 and Comparative Example 1 were cut into 4×4 cm² pieces andthen the amount of active material was analyzed. The amounts found were150 mg in Example 1 and 120 mg in Comparative Example 1. Table 2 belowshows electrode density, tap density and measured capacity of the activematerial. From Table 2, electrode density and tap density of the activematerial of Example 1 were larger than those of Comparative Example 1,and the specific capacity of Example 1 is higher than that ofComparative Example 1.

TABLE 2 Electrode Tap Specific Specific Cell density density capacity atcapacity at 0.5C capacity (g/cm³) (g/cc) 0.2C (mAh/g) (mAh/g) (mAh) Ex.1 3.79 2.5 160 150 24 Com. 3.42 2.1 160 143 19.2 Ex. 1

Hereinafter, the structure and components of the surface-treatment layerwill be explained.

Analysis of Structure and Components of Surface-Treatment Layer

Active material prepared according to Example 1 of the present inventionhas a surface-treatment layer comprising a solid-solution compoundincluding Al and P and an AlPO_(k) (k is 2 to 4) compound on the surfaceof the active material. In order to confirm the presence of thesurface-treatment layer, elemental mapping was performed on the surfaceof the cross-section of a grain of the surface-treated active materialusing STEM (Scanning Transmission Electron Microscopy). The results areshown in FIGS. 11A to 11E. FIG. 11A is a STEM photograph of activematerial in a bright field, and FIGS. 11B to 11E are STEM photographsshowing distribution of Co, Al, P and O respectively. As shown in FIGS.11B to 11E, Co, Al, P and O are all found in the surface portion of theparticle, suggesting the existence of the solid-solution compound andthe AlPO_(k) (k is 2 to 4) compound.

In order to analyze components of the surface-treatment layer formed onthe surface of the active material prepared according to Example 1, ananalysis for Al, O, Co, and P was carried out using Auger spectroscopy.FIG. 12 shows the result from the surface to a depth of about 380 Å.FIG. 12 shows that a layer of the solid-solution compound including Aland P and another layer of the AlPO_(k) (k is 2 to 4) compound wereformed from the surface to a depth of about 230 Å, and CoO₂ (possiblyLi_(1-x)CoO₂ where x is greater than or equal to 0.5) was formed furtherinside.

In order to estimate distribution of various elements through the bulkof the particle, Electron Probe Micro Analysis (EPMA) for Co, Al, and Pwas performed by line scanning across the cross-section of a particlegrain of the active material prepared in Example 1. FIGS. 13A and 13Bshow the results. As shown in FIG. 13B, the presence of Al and P wasfound only in the surface layer of the particle at less than 1 μm indepth. These results indicate that the solid-solution compoundsincluding Al and P and the AlPO_(k) (k is 2 to 4) compound in thesurface-treatment layer did not diffuse further into the bulk of theactive material.

FIG. 14 shows the results of analysis for the solid-solution compoundincluding Al and P and AlPO_(k) (k is 2 to 4) compound ofsurface-treatment layer of the active material prepared in Example 1using X-ray photoelectron spectroscopy (XPS). From FIG. 14, it can beconfirmed that peak positions of 0 and P in the surface-treated activematerial agrees well with that of P₂O₅, which indicates that a doublebond of P═O exists in the solid-solution compound and the AlPO_(k)compound. However, electrochemical characteristics of the solid-solutioncompound and the AlPO_(k) (k is 2 to 4) compound are not identical tothose of the P₂O₅ compound. For example, the capacity of the cell ofComparative Example 3 comprising active material coated with P₂O₅ on thesurface deteriorates rapidly with high-rate (1C) cycling, while the cellof Example 1 containing the active material comprising thesolid-solution compound and the AlPO_(k) (k is 2 to 4) compoundmaintains good capacity and average voltage on cycling both at a highrate as well as at a low rate (see FIG. 9). This is probably because,although the solid-solution compound and the AlPO_(k) compound ofExample 1 and the P₂O₅ compound have a double bond in the surface layerof the active material, the P₂O₅ compound is different from thecompounds in the surface-treatment layer in terms of effect on themobility of lithium ions. That is, the solid-solution compound and theAlPO_(k) compound that have a double bond probably promote the mobilityof lithium ions so that the capacity at the high rate can be maintainedat a high level.

In order to verify the effect of the AlPO_(k) compound upon the mobilityof lithium ions, oxidation and reduction peaks of cyclic voltammogramsof the cells of Comparative Example 1 and Example 5 were studied. Thecyclic voltammograms were measured in the voltage range of 3V to 4.4V ata scanning rate of 0.02 mV/sec. Lithium metal was used as the referenceelectrode in the cell. FIGS. 15A and 15B show the results. The widths ofoxidation/reduction peaks in the cyclic voltammogram of Example 5 aresignificantly smaller than those of Comparative Example 1, indicatingthat the electrode reaction rate is improved, and therefore, themobility of lithium ions is also improved by the surface layer.

The fact that the mobility of lithium ions in the compound formed on thesurface of the present positive active material is high was confirmed bythe measurement of diffusion coefficient of lithium ions as shown inFIG. 16. The diffusion coefficient of lithium ions for Example 5 of thepresent invention is five times as high as that of Comparative Example1.

In addition, the cyclic voltammograms of FIG. 15A for ComparativeExample 1 indicate that a phase transition occurs from a hexagonal phaseto a monoclinic phase and then it returns to the hexagonal phase atabout 4.1 to 4.25V. On the contrary, cyclic voltammograms of Example 5(FIG. 15B) have no peak that is assumed to be related to this phasetransition.

The reason that positive active material of the present invention doesnot show a peak that is assumed to be related to the phase transition inthe cyclic voltammogram is because the c-axis, which affects volumeexpansion during charging, hardly changes. Changes of lattice constants(a-axis and c-axis) of active materials prepared according to Example 5and Comparative Example 1 during charge-discharge in the voltage rangeof 4.6V to 4.25V were measured. FIGS. 17A and 17B show the results.Active material of Comparative Example 1 shows phase transitions fromhexagonal (H) to monoclinic (M) and back to hexagonal (H) phase in thevoltage range of 4.1 to 4.25V during charge-discharge. The change of thec-axis lattice constant was larger than that of the a-axis. When theseanisotropic contraction and expansion are more than 0.2% of theelasticity limit of the active material, micro-cracks develop in theparticles so that the particles break down to smaller particles causingthe decrease of the cell capacity. As indicated in FIG. 17A, the activematerial of Comparative Example 1 had contraction and expansion of morethan 1% in the c-axis so that micro-cracks occurred in the particles andthus the cell capacity decreased sharply as the discharge rate increased(see FIGS. 1 and 7). On the contrary, the cell of Example 5 showed asignificantly reduced variation of lattice constant of the c-axis (FIG.17A) so that the cell capacity was maintained high at the high rates ofdischarge (see FIGS. 1, 7 and 8).

Evaluation of Thermal Stability

In order to evaluate thermal stability of the positive active materialprepared according to Example 5 of the present invention and ComparativeExample 1, DSC analysis was performed as follows. The coin cells ofExample 5 and Comparative Example 1 were charged using a voltage cut-offat 4.3V. About 10 mg of the positive active materials from chargedelectrodes of each cell were collected. DSC analyses were carried out insealed aluminum cans using a 910 DSC (TA Instrument company) equipmentby scanning temperatures from 25 to 300° C. at the rate of 3° C./min.The results are shown in FIG. 18.

As shown in the FIG. 18, Comparative Example 1 (not surface-treatedLiCoO₂) showed a large exothermic peak in the temperature range of 180to 220° C. as a result of O₂ release from the breakage of Co-0 bonds ofcharged Li_(1-x)CoO₂ followed by the exothermic reaction of the oxygenwith electrolyte. This phenomenon is known as the cause of the safetyproblem. However, in the case of Example 5, the exothermic peak in theDSC was reduced to a negligible size, strongly suggesting that thethermal stability, and therefore the safety of the batteries using theactive material of Example 5 will be much better than that ofComparative Example 1.

In order to evaluate the thermal stability of the active material as afunction of the heat-treatment temperature and time, the DSCmeasurements of the charged electrodes at the cut-off voltage of 4.3Vfor the cells of Examples 6 and 7 were carried out as shown in FIG. 19.The open circuit voltage (OCV) of the cell disassembly was maintained atover 4.295V. As shown in FIG. 19, the electrode samples from the cellsof Examples 5 to 7 showed excellent thermal stability.

After overcharging cells of Examples 1 and 8 at 4.6V, thermal propertiesof the charged cells were measured by the same method as in the case of4.3V-charge. Results are shown in FIGS. 20 and 21. The cells of Examples1 and 8 which were charged at 4.6V showed negligible exothermic heatevolution while those of Comparative Examples 1 and 2 which are chargedat the same voltage showed large exothermic peaks indicating excellentthermal stability of the positive active material of the presentinvention.

If LiCoO₂ is overcharged at voltages over 4.93V, it turns to a Cdl₂-typehexagonal structure that does not have Li. This structure has the leastunstable state of the active material and it reacts rapidly withelectrolyte at high temperature, releasing oxygen. After overchargingthe cell of Example 8 at 5V, DSC measurement was carried out. The resultis shown in FIG. 22. The DSC measurements of the cells of Examples 1 and9 were also carried out. The results are shown in FIG. 23. As shown inFIGS. 22 and 23, in the case of overcharging to 5V, the cells ofComparative Examples 1 and 2 that do not have the surface-treatmentlayer show a low starting temperature (T_(s)) of the exothermic reactionand a relatively large exothermic heat, and multiple exothermic peakswere observed. Although active materials after overcharging the cells ofExamples 1, 8 and 9, which contained positive active material coatedwith the solid-solution compound and the AlPO_(k) (k is 2 to 4)compound, at 5V showed exothermic peaks unlike the case of the 4.3V- and4.6V-charge, the starting temperature of the exothermic reaction wasover 235° C. which is more than 40° C. higher than that of conventionalLiCoO₂ (Comparative Examples 1 and 2) without the surface treatment.

The coated active material, LiCoO₂, according to the present invention,has comparable or better thermal stability and characteristics (Examples1 and 9), as shown in FIG. 23, than uncoated LiMn₂O₄ (ComparativeExample 4) active material which is known for its good thermal stabilityand characteristics among positive materials used in commerciallyavailable lithium secondary battery cells.

In order to evaluate thermal stability of positive active materials ofExample 10 and Comparative Example 4 (not surface-treated LiMn₂O₄), DSCmeasurements were carried out using a similar technique as describedabove. The results are shown in FIG. 24. Even without thesurface-treatment, LiMn₂O₄ is known as the most thermally stable ofpositive active materials. The exothermic heat of active materialcontaining the surface-treatment of the AlPO_(k) compound is remarkablyreduced from that of the untreated material indicating that the thermalstability is remarkably improved by the surface-treatment.

FIG. 25 shows the results of a DSC evaluation of the thermal stabilityof positive active materials of Examples 12 and 13 and ComparativeExample 5 (not surface-treated LiNi_(0.8)Mn_(0.2)O₂). The cells ofExamples 12 and 13 and Comparative Example 5 were overcharged at 4.6V.Fully charged positive electrode samples were taken from the cells.About 10 mg of the active material was removed from the electrodesamples. The test material was sealed in an aluminum can and DSCthermograms were obtained using 910 DSC (TA Instrument company)equipment by scanning temperatures from 100 to 300° C. at the rate of 3°C./min. As shown in FIG. 25, the values of the exothermic heat of thesurface-treated LiNi_(0.8)Mn_(0.2)O₂ active material according toExamples 12 and 13 decreased to about 1/30 of the value of the activematerial of Comparative Example 5 that was not surface-treated.Therefore, the thermal stability of the nickel manganese-based activematerial is improved significantly, similar to the cases of thecobalt-based and manganese-based active materials.

Twenty 930 mAh prismatic Li-ion cells comprising the positive activematerials prepared in Examples 1, 8 and 9, and Comparative Examples 1(conventional, not surface-treated, LiCoO₂) and 4 (conventional, notsurface-treated, LiMn₂O₄) were safety tested for the categories ofcombustion, heat exposure and overcharge. The sample Li-ion cells wereprepared by the same method as in Example 15. The combustion test wasperformed by heating cells using a burner, and measuring the percentageof the cells that underwent an explosion. The heat exposure test wasperformed by exposing the cells to 150° C. measuring times before anexplosion time was measured. The overcharge test was carried out byobserving the percentages of the sample cells that were overcharged at1C rate. These results are presented in Table 3.

TABLE 3 Com. Ex. 1 Com. Ex. 4 Ex. 1 Ex. 8 Ex. 9 Combustion 100% 50% 0%0% 0% test (Explosion percentage) Heat 5 min. 20 min. 90 min. 95 min. 95min. exposure test (Average explosion Time) Overcharge test  90% 30% 0%0% 0% (Leak percentage)

For the above safety tests for 930 mAh Li-ion cells comprising positiveactive materials of Examples 1, 8 and 9 and Comparative Examples 1 and4, the overcharge test was carried out at three different charge rates,1C, 2C, and 3C, as shown in Table 4. Five cells were tested for eachcharge rate.

TABLE 4 C-rate Com. Ex. 1 Com. Ex. 4 Ex. 1 Ex. 8 Ex. 9 1C 5L5 5L0 5L05L0 5L0 2C 5L5 5L5 3L0, 2L4 3L0, 2L4 3L0, 2L4 3C 5L5 5L5 4L3, 1L4 4L3,1L4 4L3, 1L4

The number preceding “L” indicates the number of tested cells.

The results of the safety test were rated as follows:

L0: good, L1: leakage, L2: flash, L2: flame, L3: smoke, L4: ignition,L5: explosion.

The temperature of the prismatic Li-ion cell of Example 15 was testedfor the increased charge voltage to 12V as shown in FIG. 26. Generally,when the cell voltage dropped to 0V during the high-voltage overcharge,the cell explodes and the cell temperature increases abruptly. However,the cell of Example 15 did not show the increase of temperature over100° C. when the charge voltage drops from 12V to 0V as shown in FIG.26. Therefore, the cells prepared using the surface-treated activematerial of the present invention show excellent safety.

The active material containing a surface-treatment layer comprising theMXO_(k) (k is 2 to 4) compound of the present invention shows excellentstructural stability and high average discharge voltages both at highand low rates, excellent cycle life characteristics, and good capacity.Its excellent thermal stability improves the safety of the cells invarious categories including combustion, heat exposure, and overchargetests. In addition, the process of the present invention uses awater-based coating liquid giving a great low cost benefit over asimilar process using an organic solvent-based solution. Since theprocess is performed at a lower temperature and in a shorter processtime than the conventional process using an organic solvent,productivity is expected to be improved in large-scale production.

The foregoing is considered illustrative only of the principles of theinvention. Further, since numerous modifications and changes willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact construction and operation shown anddescribed. Accordingly, all suitable modifications and equivalents maybe resorted to that fall within the scope of the invention and theappended claims.

What is claimed is:
 1. An active material for a battery, comprising: anelectrochemically reversibly oxidizable and reducible base materialselected from the group consisting of a lithium-containing metal oxide,and a lithium-containing chalcogenide compound; and a surface-treatmentlayer formed on the base material and comprising a solid-solutioncompound including surface treating elements M and X, and a compound ofthe formula MXO_(k), wherein: M is at least one element selected fromthe group consisting of an alkali metal, an alkaline earth metal, agroup 13 element, a group 14 element, a transition metal, and arare-earth element, X is an element that is capable of forming a doublebond with oxygen, and k is a numerical value in the range of 2 to 4, andan oxide comprising Li, M, and Co being present between thesurface-treatment layer and the base material.
 2. The active material ofclaim 1, wherein the element M is selected from the group consisting ofNa, K, Mg, Ca, Sr, Ni, Co, Si, Ti, B, Al, Sn, Mn, Cr, Fe, V, Zr, and acombination thereof.
 3. The active material of claim 1, wherein theelement M is Al.
 4. The active material of claim 1, wherein the elementX is selected from the group consisting of P, S, W, and a combinationthereof.
 5. The active material of claim 1, wherein the element X is P.6. The active material of claim 1, wherein the oxide is represented byLi_(1-x)AlCoO₂ wherein 0.5≦x and 0<y≦1.
 7. The active material of claim1, wherein an amount of the element M is 0.9 to 6 at % and an amount ofthe element X is 0.6 to 5 at % in the solid-solution compound at asurface of the active material to a depth of about 230 Å.
 8. The activematerial of claim 1, wherein a concentration of the elements M and Xdecreases gradually from a surface to a center of a particle grain ofthe active material.
 9. The active material of claim 1, wherein anamount of the Co is 14 to 29 at % in the solid-solution at a surface ofthe active material to a depth of about 230 Å.
 10. The active materialof claim 1, wherein a total amount of the element M in the activematerial is 0.1 to 15% by weight of the active material.
 11. The activematerial of claim 1, wherein a total amount of the element M in theactive material is 0.1 to 6% by weight of the active material.
 12. Theactive material of claim 1, wherein a total amount of the element X is0.1 to 15% by weight of the active material.
 13. The active material ofclaim 1, wherein a total amount of the element X in the active materialis 0.1 to 6% by weight of the active material.
 14. The active materialof claim 1, wherein a thickness of the surface-treatment layer is 0.01to 2 μm.
 15. The active material of claim 1, wherein a tap density ofthe active material is 1 to 3 g/cc.
 16. The active material of claim 1,wherein the base material is at least one selected from the groupconsisting of lithium compounds represented by formulas (2) to (14):Li_(x)Mn_(1-y)M′_(y)A₂  (2)Li_(x)Mn_(1-y)M′yO_(2-z)B_(z)  (3)Li_(x)Mn₂O_(4-z)B_(z)  (4)Li_(x)Mn_(2-y)M′_(y)A₄  (5)Li_(x)Co_(1-y)M′_(y)A₂  (6)Li_(x)Co_(1-y)M′_(y)O_(2-z)B_(z)  (7)Li_(x)Ni_(1-y)M′_(y)A₂  (8)Li_(x)Ni_(1-y)M′_(y)O_(2-z)B_(z)  (9)Li_(x)Ni_(1-y)Co_(y)O_(2-z)B_(z)  (10)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)A_(α)  (11)Li_(x)Ni_(1-y-z)Co_(y)M′_(z)O_(2-α)B_(α)  (12)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)A_(α)  (13)Li_(x)Ni_(1-y-z)Mn_(y)M′_(z)O_(2-α)B_(α)  (14) wherein 0.95≦x≦1.1,0≦y≦0.5, 0≦z≦0.5, and 0α≦2; M′ is at least one element selected from thegroup consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and a rare-earthelement; A is at least one element selected from the group consisting ofO, F, S and P; and B is at least one element selected from the groupconsisting of F, S and P.
 17. The active material of claim 1, whereinthe base material is at least one selected from the group consisting oflithium compounds represented by formulas (6) to (7):Li_(x)Co_(1-y)M′_(y)A₂  (6)Li_(x)Co_(1-y)M′_(y)O_(2-z)B_(z)  (7) M′ is at least one elementselected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V,and a rare-earth element; A is at least one element selected from thegroup consisting of O, F, S and P; and B is at least one elementselected from the group consisting of F, S and P.